Many properties of chemical elements vary periodically throughout the periodic table. It's the calls periodic properties.
The periodic properties can be:
- atomic radius
- atomic volume
- absolute density
This fact obeys the Moseley Law of Periodicity: "Many physical and chemical properties of elements vary periodically following their atomic numbers."
Properties where values only increase with atomic number and others where values only decrease we call aperiodic properties. Example: the specific heat.
The atomic radius of the elements is a periodic property, since their values vary only periodically, that is, they increase and decrease with the increase of the atomic number.
The atomic radius (r) is half the minimum internuclear distance (d) that two atoms of this element can have without being chemically bonded.
To measure the atomic radius, the x-ray diffraction technique is used.
In a family of the periodic table, the atomic radius increases from top to bottom and in the period increases from right to left.
For this rule noble gases are not allowed, since they have the largest atomic radius in each period.
Looking at the periodic table, we can verify that the francius (Fr) has the largest atomic radius.
If the atom becomes a cation ion or anion, its radius changes.
- The radius of the atom is always greater than the radius of its cation ion because it loses electrons.
- The radius of the atom is always smaller than the radius of its anion ion because it gets electrons.
Atomic volume is a periodic property because it varies periodically with increasing atomic number.
Atomic volume is the ratio between the mass of an amount of matter (1 mol = 6.02.1023 atoms) and the density of the simple substance formed by this element in the solid phase.
It is not the volume of an atom, but of a set of atoms. In atomic volume, it influences not only the volume of each atom, but also the spacing that exists between these atoms.
In the periodic table, atomic volume values increase from top to bottom in families and, in a period, from center to table ends.